In anaesthesia, with the patient connected to a respirator, it is often desirable to monitor the alveolar gases of a patient for their composition and pressure as well as flow and volume quantities.
The flow and volume quantities are often monitored through use of a spirometer which is connectable to an intubation tube. One common type of spirometer is a so-called pneumotachometer which provides a slight constriction in the flow path, whereby the flow rate can be calculated on the basis of a pressure difference created thereover. However, the measurement is rather inaccurate for several reasons, the most significant of which is the dependence of the pressure difference on the viscosity of a gas, i.e., actually on the composition of a gas. The pneumotachometer can be fitted in a patient tubing as a disposable product, which aids in eliminating sterility problems. Otherwise, the pneumotachmeter would be quite an attractive flow sensor thanks to its simplicity.
The composition of gases is often determined by directing a flow sample from the mouth of a patient into a gas analyzer.